The present invention relates to techniques for drilling oil, gas, water, geothermal, or analogous wells. More precisely, the invention relates to cementing compositions which are suitable for cementing zones which are subjected to extreme dynamic stresses.
In general, a well which is more than a few hundreds of meters deep is cased, and the annular space between the underground formation and the casing is cemented over all or a portion of its depth. The essential function of cementing is to prevent fluid exchange between the different formation layers through which the hole passes and to control the ingress of fluid into the well, in particular to limit the ingress of water. In production zones, the casing, the cement and the formation are all perforated over a depth of a few centimeters.
The cement positioned in the annular space in an oil well is subjected to a number of stresses throughout the lifetime of the well. The pressure inside the casing can increase or decrease as the fluid filling it changes or as additional pressure is applied to the well, such as when the drilling fluid is replaced by a completion fluid or by a fluid used in a stimulation operation. A change of temperature also creates stress in the cement, at least during the transition period before the temperatures of the steel and the cement come into equilibrium. In the majority of the above cases, the stressing process is sufficiently slow to enable it to be treated as a static process.
However, the cement is subjected to other stresses which are dynamic in nature either because they occur over a very short period or because they are either periodical or repetitive to a greater or lesser extent. Perforating does not just cause an over-pressure of a few hundred bars inside the well which dissipates in the form of a shock wave. In addition, perforating creates a shock when the projectile penetrates the cement and that shock subjects the zone surrounding the hole to large forces extending over a length of a few meters.
Another process which creates dynamic stresses in the cement and which is now very common in oilwell operations is when a window is cut in a cemented casing to create a sidetrack. Milling the steel over a depth of a few meters followed by drilling a sidetrack subjects the cement to shock and to vibration which often damage it irreversibly.
The present invention aims to provide novel formulations, in particular for cementing regions in oil or analogous wells which are subjected to extreme dynamic stresses such as perforation zones and junctions for branches of a sidetrack.
In an article presented at the SPE (Society of Petroleum Engineers) annual conference and exhibition 1997 (SPE 38598, 5-8 Oct. 1997) and in French patent application FR 97 11821, 23rd Sep. 1997) Marc Thiercelin et al. have shown that the risk of rupture of a cement sheath depends on the thermoelastic properties of the casing, on the cement, and on the formation which surrounds the well. A detailed analysis of the mechanisms leading to rupture of the cement sheath has shown that the risk of rupture of a cement sheath following an increase in pressure and/or temperature in the well is directly linked to the tensile strength of the cement and is attenuated when the ratio of the tensile strength RT of the cement over its Young's modulus E is increased.
The aim of the present invention is to provide lighter oilwell cements reinforced with recycled rubber. The ground rubber particles reduce the density of the slurry and thus, secondarily, affect the flexibility of the system; primarily, the rubber particles do not improve the mechanical properties of the cements.
In the building industry, including rubber particles in concrete is known to improve toughness, durability, and resilience [see, for example, N. N. Eldin and A. B. Sinouci, Rubber-Tire Particles as Concrete Aggregate, Journal of Materials in Civil Engineering, 5, 4, 478-497 (1993)]. Concretes including rubber particles in their formulations have applications, for example, in motorway construction to absorb shocks, in anti-noise walls as sound absorbing material, and also in constructing buildings to absorb seismic waves during earthquakes. For those applications, the aim of the rubber particles is thus essentially to improve the mechanical properties of the concrete.
The addition of ground rubber particles (with grain size in the 4-20 mesh range) is known in the oilwell cement industry [Well cementing 1990, E. B Nelson, Schlumberger Educational Services] to improve impact strength and bending strength. Such an improvement in mechanical properties is also indicated in Russian patents SU-1384724 and SU-1323699. More recently, U.S. Pat. No. 5,779,787 has proposed the use of particles derived from recycling automobile tires, with grain sizes in the 10/20 mesh (850-2000 μm) or 20/30 mesh (600-850 μm) range, to improve the mechanical properties of hardened cements, in particular their resilience, ductility, and expansion properties. It should be noted that the densities of the slurries disclosed in that American patent were in the range 1.72 g/cm3 to 2.28 g/cm3.